1. Field of the Invention
The present invention relates to a novel, pulp-like acrylic short fiber. More particularly, the invention relates to a novel, heat- and chemical-resistant acrylic short fiber produced by melt extruding polyacrylonitrile (hereinafter, referred to as PAN) hydrate followed by heat stabilizing the resulting extrudate without spinning.
2. Description of the Prior Art
Acrylic fibers have been spotlighted as materials of clothings as well as, more recently, industrial materials such as substitute fibers for asbestos, heat insulating and resisting fibers, cement reinforcing fibers and the like. Such acrylic fibers to be used as industrial materials should, however, be produced in the form of a short fiber.
Short fibers have hitherto been produced in the form of a staple by solution spinning methods wherein a solvent is used and the resulting fibers are drawn to form long fibers followed by cutting the long fibers into staples.
Such prior art methods for producing short fibers suffer from the defects that due to the use of a solvent, various complicated steps of extracting, recovering and purifying the solvent, and preventing an environmental pollution are essentially involved; thus, an economic load undertaken is very large, and environmental pollution problems may have also been caused. Furthermore, short fibers in the form of a staple cannot fully satisfy various properties and characteristics required in an industrial material, such as reinforcing, heat insulating and binding properties.
According to prior art processes for the preparation of acrylic fibers, fibers having molecular orientation could not have been prepared without filament spinning through microholes followed by drawing in a high draw ratio. Furthermore, molecular-oriented, pulp-like fibers could have been prepared only by complicated processes comprising various steps of preparing a spinning solution, spinning, solidifying, removing and recovering a solvent, drawing, cutting, fibrillating and the like.
It is well known that PAN molecular chains are twisted into the form of an irregular helix due to strong polarity of nitrile groups in the side chains thereof and have characteristics closely allied to rigid chains; see W. R. Krigbaum et al., Journal of Polymer Science, Vol. XLIII, pp 467-488 (1960). If a strong polar solvent, such as dimethylformamide, dimethylacetamide, dimethylsulfoxide, or aqueous NaSCN solution, aqueous ZnCl.sub.2 solution or aqueous HNO.sub.3 solution is added to such PAN, the nitrile groups attract the molecules of these solvents to combine therewith, and thereby the groups are separated from each other to form a fluid solution even at room temperature.
If the resulting fluid solution is extruded through microholes in a spinning die, and then the solvent is removed, PAN is solidified to take the form of a fiber. However, molecular chains in the solidified PAN still form an original, non-oriented lump in which they are bound with each other. Therefore, the resulting filaments are deemed to be present in the form of a fiber immediately after spinning. However, if the solvent is removed and then the filaments are dried, the PAN molecular chains in the filaments are reconglomerated to form a non-oriented lump after all since internal molecular chains in the resulting filaments have not been oriented at all. Accordingly, in order to obtain a complete fibrous structure from the viewpoint of molecular construction, it is necessary to draw the resulting filaments in a high draw ratio of above 5 to 30 so that the molecular chains are arranged in parallel with the fiber axis. As the filaments are drawn, the conglomerated PAN molecular chains are become disentangled and extended out while arranging with each other, resulting in the formation of the fibers having an extended chain crystal region. As discussed above, the step of drawing is indispensable in the prior art techniques for producing fibers, and therefore the substantial fiber structure in which most molecular chains are oriented in parallel with the fiber axis cannot be obtained until the resulting filaments are subjected to drawing.
Various processes for the preparation of fibers which comprise forming a melt by heating a mixture of PAN and water, followed by extruding the resulting melt have been proposed, for example, in U.S. Patent No. 2,585,444. However, according to such processes, for the easier spinning of a melt, it is necessary to lower the viscosity of the melt. Therefore, the amorphous melt cannot help being obtained at a high temperature at which a crystalline phase is broken down, and then the resulting melt is subjected to spinning. Thus, PAN molecular chains oriented in parallel with each other cannot be obtained until the resulting filaments are subjected to drawing in a high draw ratio.
For example, U.S. Pat. No. 2,585,444 teaches that PAN fibers can be produced by heating PAN hydrate containing 30% to 85% by weight of water to a temperature above its melting point to give a melted fluid followed by melt spinning the resulting fluid. U.S. Pat. Nos. 3,896,204 and 3,984,601 disclose processes for the production of fibers which comprise heating a mixture of PAN and water of about 20% to 30% by weight to a temperature ranging from 170.degree. C. to 205.degree. C. to give an amorphous melt, and spinning the resulting melt followed by drawing in a draw ratio of 5 or more to obtain fibers. The above patents also teach that if the content of acrylonitrile in PAN is as low as 80%, the step of spinning can be carried out at a temperature ranging from 140.degree. C. and 170.degree. C. However, as can be seen in FIG. 3, since the higher the content of a comonomer except for acrylonitrile is, the lower the temperature at which amorphous melt is formed is, PAN containing about 20% by weight of the comonomer can be transformed into an amorphous melt even at 140.degree. C.
Therefore, within the temperature range and under the spinning conditions mentioned above, a melted metacrystalline phase having a highly-oriented molecular structure cannot be obtained.
U.S. Pat. Nos. 3,991,153 and 4,163,770 disclose processes for the production of fibers which comprise spinning PAN hydrate containing 10% to 40% by weight of water at temperatures above its melting temperature, that is, the temperature range in which a melt of an amorphous, single phase is formed, and then drawing the resulting extruded filaments in a draw ratio of 25 to 150 in a pressure chamber. In these cases, since PAN molecular chains in the melt exist in an irregular and random state, a fibrous structure is not formed until drawing is applied in a high draw ratio.
As mentioned above, conventional processes typically involve the steps of forming and spinning a PAN/H.sub.2 O melt. However, since the spinning is carried out in the temperature range in which all melts exist in a random state, good orientation of PAN molecular chains cannot be obtained until the extruded filaments are drawn in a high draw ratio.
U.S. Pat. Nos. 3,402,231, 3,774,387 and 3,873,508 also disclose processes for the production of fibers for pulp, which comprise adding water of 100% or more to PAN to form a PAN mixture, heating the resulting mixture at about 200.degree. C. to form a melt, and then spinning the resulting melt to produce fibers. However, since in these patents an excess of water and elevated temperatures are used in order to obtain the PAN mixture, the resulted PAN/H.sub.2 O melt takes a random, amorphous form as well as PAN filaments extruded therefrom are no more than a non-oriented, continuous foam which does not practically have neither the orientation of molecular chains nor a fibrous structure, although they appear to be formed into the form of a fiber externally.
As mentioned above, the conventional techniques characterized by melt spinning of PAN hydrate have been dependent on a usual process which comprises forming an amorphous melt by using an excess of water, or by heating PAN hydrate to temperatures above its melting temperature, or by increasing the content of a comonomer, spinning the resulting amorphous melt to form filaments, and then drawing the resulting filaments in a high draw ratio to form fibers.